During the past week, clients have seen payments to go awry, holiday and home purchases interrupted and wages have appeared to go missing.

NatWest has more than 7.5 million personal banking customers but it remains unclear how many have been affected.

More than 1,000 NatWest branches are opening their doors from 8am to 7pm as staff seek to resolve the problems.

Susan Allen, director of customer services at RBS Group, insisted that progress was being made and expressed cautious optimism that RBS and NatWest customer account balances would be largely "back to normal" at the start of the working week.

"The knock-on effects of this technical failure mean there will be bumps in the road," she said. "We will do everything we can to minimise further disruption to our customers."

Stephen Hester, chief executive of NatWest owner RBS, has issued a public apology for the chaos and conceded the bank had let down its customers. His attempt to reassure clients followed mounting fears that thousands of people could be hit with penalty charges if their regular payments - including mortgages - were affected.